1. Field of the Invention
This invention relates to emission control of internal combustion engines. In particular, the invention relates to on-board monitoring of an efficiency of a Three Way Catalytic converter (TWC) installed on a vehicle to minimize emission from an engine.
2. Prior Art
It is known in the field that catalytic conversion relates to a catalytic oxygen storage capability. A properly operating TWC converter dampens exhaust gas component fluctuations in the exhaust stream. One proposed system (see SAE paper 900062, Detection of catalytic efficiency loss using on-board diagnostic) employs two exhaust gas oxygen (EGO) sensors, one upstream and one downstream of the converter, to detect those changes in oxygen content before and after the converter. The system further employs test signals in a form of an air-fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. By comparing the change in response patterns between pre- and post-converter EGO sensors, a determination can be made about catalytic efficiency. A deficiency of that method is that during the test the fuel control system operates in an open loop control mode, and air-fuel ratio tends to shift from stoichiometry. Test results also depend on two EGO sensors with different characteristics due to manufacturing tolerances or aging which may lead to additional errors. Further, the particular selected air-fuel ratio swing and frequency greatly influence results of the test. The problem and disadvantages discussed above are overcome by this invention.